In second trimester of pregnancy
due to increase in foetal size, abdominal respiration is affected and now the respiration is mainly thoracic. As a result thorax moves more to fulfil the requirement of respiration. On laying down abdominal content further presses the thorax causing more laboured breathing and pain
. It is common. Avoid exertion, sleep in propped up position. If pain is unbearable you may get your ultrasound
done to rule out any lung effusion.